The present invention relates generally to controlling an analog-front-end/digitizer having a plurality of color-signal-processing channels. The invention further relates to controlling the analog-front-end/digitizer such that each color component of a serial-analog-color signal generated by a contact image-sensor scan head is individually processed by a respective gain/offset channel
FIG. 1 illustrates certain components of a conventional scanner 10. The scanner 10 can be part of a copier or a device used for digitizing images for use with a personal computer. The scanner 10 typically includes a glass platen 14 on which a source image 12, such as a document or photograph, is placed for scanning. The scan head 20 typically includes a white light 22 for illuminating the source image 12, and is moved relative to the source image in a direction 16. The scan head 20 also includes a relatively small, i.e., perhaps one inch by one inch, charged coupled device (hereafter CCD) 28, which captures an image of the source image 12. A lens 26 and an optical path 23 reduce the size of an image of the source image 12, which may be 8½-inches wide where the source image is a standard size sheet of paper, down to a size capturable by the CCD 28. An optical path of at least one foot is usually required for this. Therefore, the scan head 20 includes a plurality of mirrors, illustrated as mirrors 24a–24c, so the path 23 does not have to be a linear foot but can be “folded down” to a shorter or compact overall dimension. The last mirror 24c reflects the image onto the lens 26, which focuses it on the CCD 28. The scan head 20, however, may have more than one lens, and more or fewer than three mirrors. Furthermore, for color images, the scan head 20 usually includes three paths 23, one for a red (23R), one for green (23G), and one for blue (23B). Portions of these paths 23R, 23G, and 23B are shown in FIG. 1.
For a color scanner 10, the CCD 28 typically has an array of three rows of sensors, one each for red (28R), green (28G), and blue (28B) (not shown). For an 8.5-inch wide, 600 dpi sensor, there are 5100 sensors in each row, for 16,300 sensors total. These arrays continuously and simultaneously capture the red, green, and blue components of the image being copied as the scan head 20 moves in direction 16 relative to the source image 12. Therefore, the CCD 28 continuously and simultaneously outputs parallel red, green, and blue analog image signals for processing into a digital representation of the source image 12.
FIG. 2 is a schematic block diagram of a conventional multiple-channel image processing-system 40 that scans images for display and use in a personal computer 46. The system 40 includes the scan head 20 of the scanner 10 of FIG. 1, and further includes paper sensors 19, a motor driver 17, and a motor 18 that moves the scan head 20 relative to the source image 12 (FIG. 1) being scanned. The system 40 also illustrates an analog front end/digitizer 42 (also referred to as “AFE 42”), a scanner controller 44, and a memory 48.
FIG. 2 is a schematic block diagram of a conventional multiple-channel image processing-system 40 that scans images for display and use in a personal computer 46. the system 40 includes the scan head 20 of the scanner 10 of FIG. 1, and further includes paper sensors 19, a motor driver 17, and a motor 18 that moves the scan head 20 relative to the source image 12 (FIG. 1) being scanned. The system 40 also illustrates an analog front end/digitizer 42 (also referred to as “AFE 42”), a scanner controller 44, and a memory 48.
The AFE 42 is known in the prior art, and includes three input channels, one each for the red, green, and blue color components of the parallel-analog signal generated by the CCD 28. Each input channel includes a respective connector 51, a programmable offset DAC 58, a SUM 52, a programmable gain amplifier 54, and a register 56. While separate registers are shown for each channel, a single register common to all three channels may be used. The outputs of each channel are coupled to a 3:1 multiplexer 60, and the multiplexer 60 is coupled to an analog-to-digital converter 62. A digital-control interface module 64 is coupled to the 3:1 multiplexer 60, the register 56, and, optionally, other components of the AFE 42. The interface 64 is configured for coupling components of the AFE 42 with devices such as the controller 44.
The controller 44 is typically an application-specific integrated circuit that includes functionality to operate the scanner 10 of FIG. 1, including the scan head 20, the AFE 42, and to interface with the personal computer 46. A memory 48 provides memory services to the controller 44, and may be any type of addressable storage device.
In use, each color-component signal generated by the sensors of CCD 28 is initially calibrated to optimize the amplitude and determine the offset. The calibration includes adjusting the amplitude of each color-component signal to use the full input range of the ADC 62. This maximizes the signal to noise ratio. For example, the Blue Analog Vout from the blue sensor 28B might have only one-half the amplitude of the Red Analog Vout from the red sensor 28R. The calibration process determines what gain is necessary for each color-component signal so that all the color-component signals will have substantially the same amplitude, and will use the full number of available bits provided by the ADC 62. Values that set the amplifiers 54 to the necessary gains are stored in the respective register 56 for each color component. Likewise, the calibration process determines the offset or dark correction necessary for each color-component analog signal, and values that set the DACs 58 to the necessary offset are stored in the respective register 56 for each color component.
Once scanning begins, the red, green, and blue sensors of the CCD 28 in response to a reflection of the white light from the source image 12 produce respective continuous and simultaneous parallel analog-color signals shown as Analog Vout in FIG. 2. The parallel Analog Vout signals are coupled respectively to the AFE 42 at connectors 51R, 51G, and 51B over lines 53R, 53G, and 53B into their respective color-component channels. The AFE 42 processes the parallel analog-color signals in their respective channels by simultaneously sampling each red, green, and blue color-component signals shown as Red Analog Vout, Blue Analog Vout, and Green Analog Vout from the sensors 28R, 28B, and 28G. The DAC 58 level shifts each color-component signal by the offset value stored in the register 56. Then, the PGAs 54R, 54B, and 54G respectively scale each color-component signal by the gain value stored in the register 56. At this point, each color component of the parallel analog-color signal has been individually processed in its channel. The three processed color signals from the three programmable gain amplifiers 54R, 54G, and 54B are then multiplexed through the 3:1 multiplexer 60, which sequentially samples the three processed color signals and generates a single analog signal that is provided to the ADC 62. The ADC 62 converts the single analog signal into a digital ADC data signal. The ADC data signal presents a single pixel at a time, and sequentially presents three colors for a single pixel column but not for a single pixel. The reason that the ADC data signal does not present three colors for a single pixel is that the physical separation of the three rows of the sensors 28R, 28G, and 28B makes the colors physically separated on the page. For example, if the red scan is from row 1, the green scan will be from row 5 and the blue scan will be from row 9—all the same pixel column number. The data (COLORrow-column) from the ADC 62 looks like:                R1-1, G5-1, B9-1, R1-2, G5-2, B9-2, . . . R1-5100, G5-5100, B9-5100        R2-1, G6-1, B10-1, R2-2, G6-2, B10-2, . . . R2-5100, G6-5100, B10-5100The ADC data signal is provided to the controller 44, which exposes the ADC data signal to the personal computer 46.        
For example, red light reflected from a source image 12 (of FIG. 1) is sensed by the red senor 28R, which generates the Red Analog Vout signal. The Red Analog Vout signal is connected by line 53R to terminal 51R of the AFE 42, where it is then connected to SUM 52R. At SUM 52R, Red Analog Vout signal is level shifted or offset by the previously calibrated red offset stored in the Red Register 56R, and the offset Red Analog Vout signal is then scaled by the PGA 54R by the previously calibrated gain. The offset and scaled Red Analog Vout signal is multiplexed through the 3.1 MUX 60 along with the green and blue offset and scaled signals and digitized by the ADC 62.
FIG. 3 illustrates certain components of a conventional scanner 70 that is similar to the scanner 10 except it includes a contact-image sensor (CIS) scan head 72. The CIS scan head 72 differs from the scan head 20 of FIG. 1 in that the CIS scan head is much more compact and, therefore allows the scanner 70 to be smaller than scanner 10. The scan head 72 is more compact because it does not require the optical path 23, the mirrors 24, or focusing provided by the lens 26 of FIG. 1. The scan head 72 has the width of the maximum source image 12, and is placed in close proximity with the glass platen 14. For example, to copy or scan an 8½-by 11 sheet of paper, the copier or scanner would include a scan head 72 that is 8½ inches wide.
The scan head 72 is in close proximity to the glass platen 14, and typically uses an array 74 of red, green, and blue light sources across the scan head 72 to provide a full spectrum of light to illuminate the source image 12. The light source typically include light emitting diodes (LED). There are two main types of illumination based on LEDs. One style includes LEDs placed across the whole width of the scan head 72, and another style includes a few (even a single per color) LEDs on the side of the scan head and a plastic wave guide or light pipe is used to distribute the light across the width of the scan head. A lens 76 is positioned between the light reflected from the source image 12 and the CCD sensor 78. A single row of sensors comprise the CCD sensor 78. The single row of sensors is distributed across the scan head 72 to receive light reflected from the source image 12 after being focused by the lens 76. In an 8.5-inch wide 600 dpi sensor, there are 5100 sensors in the single row. While the scan head 20 of FIGS. 1 and 2 uses a single white light source 22 and three different CCD sensors 28R, 28G, and 28B on a small chip to capture the color components of the source image 12, the CIS scan head 70 uses three different colored light-source arrays 74R, 74G, and 74B, and a single CCD sensor 78 both distributed across a width of the scanned source image 12 to capture all three color components.
To scan the source image 12, for example, first the red-light source(s) 74R are flashed across the width of the source image 12 to illuminate a line of the source image and provide the red components of the image. Then, the green-light source(s) 74G are flashed to provide the green components, and then blue-light source(s) 74B are flashed to provide the blue components. During each light flash, light reflected from the source image 12 is focused by the lens 76 onto the CCD sensor 78, which captures the color component of the image and outputs a representation analog signal. Each color light source 74 is sequentially flashed once for each line and the CCD sensor 78 serially generates the analog color signals in the sequence that the lights are flashed, in this example red, green, and blue. This cycle continues such that the red, green, and blue components of each line of the source image 12 are scanned. Although the scan head 72, with its light array 74 and CCD sensor 78, may move step-by-step so as to scan one line three times, once each for the RGB color components, it is more common for the scan head 72 to move at a constant velocity such that the red, green, and blue components are each scanned for one of three overlapping lines. Since the constant velocity allows every 3rd scan to be a new line (every 3rd scan is the same color), the scanner has only moved or stepped ⅓ of an overlapping line for each color scan. Therefore, the three scanned colors are overlapping. For each line sampled by the CCD scanner 10 of FIG. 1, the CIS scanner 70 of FIG. 3 will scan three overlapping lines. With an equal number of sensors in each row, the resolution of the CCD and CIS type scanners is the same.
The analog signal produced by the CCD sensor 78 of the CIS scanner 70 as it scans RGB color components serially is referred to herein as a serial analog-color signal. The serial analog-color signal contrasts with the three channel analog signal provided by the three rows of sensors of the CCD 28 of the scanner 10 of FIG. 1, which generates the three-color components in parallel on three parallel conductors. Commonly available analog front end/digitizers, such as the AFE 42 illustrated in FIG. 2, do not readily provide a circuit or method for individually processing each color component of a serial-analog signal generated by a CIS scan head 72 to adjust offset and gain. One proposed compromise solution is to couple the CCD sensor 78 to one channel of a commonly available AFE, such as to the red channel at connection 51R of the AFE 42 of FIG. 2, and establish a single-offset value and a single-gain value. The single values would be applied to all three colors. The proposed solution is not adequate because the single values do not take into account a potential for significant variation in the red, green, and blue color-component signals generated by the CCD sensor 78.